Some ppl say dreams are just your thoughts...but some say that some dreams tell the future or sumthing. O.o exactly what are they??

Also, I remember dreaming about something a long time ago...and then i actually see some of it happen right around this month. I visited some new store at the mall....and I actually dreamt of doing that before the store was even open. O.O ...now thats rlly creeping me out.